A customer tried to explore JEbC using this metaplugin but found that it (a) installed every JEbC plugin, which he did not necessarily want; (b) forced installation/upgrade of all OSS plugins that JEbC currently happens to bundle, even those which he already had installed in a version he had tested and liked.

It would be better when installing the metaplugin to explicitly ask what you want to do. One option would be the current behavior, which is to add/upgrade everything, similar to what would happen if you started running the JEbC WAR file on the same $JENKINS_HOME (though not identical, since that would currently leave your pinned plugins alone: JENKINS-24046). But another option would be much more conservative:

1. Download async-http-client from the OSS update center and schedule for installation after restart, unless you have it already in a sufficiently new version. If you have it installed but it is too old, download the current version and schedule for update after restart.
1. Download versions of the cloudbees-license and nectar-license plugins from the JEbC update center matching your core version, and schedule them for installation after restart.
1. Restart Jenkins, which should automatically switch your default update center to the correct JEbC one.

At that point you could install or update any other plugins on your own schedule.